1. Field of the Invention
The present invention concerns a stereolithography method and a stereolithography machine employing said method.
2. Present State of the Art
As is known, the stereolithography technique is used to produce three-dimensional objects through the successive deposition of several layers of a light-sensitive liquid resin capable of polymerizing through exposure to a light source.
In particular, each resin layer is superimposed to the preceding layer of the three-dimensional object to be produced and is allowed to selectively solidify in the points corresponding to the volume of the object itself.
A stereolithography machine according to a known embodiment comprises a container suited to contain said liquid resin and provided with a transparent bottom.
There is also a light source, generally a laser emitter or a projector, capable of selectively irradiating the layer of liquid resin arranged adjacent to the bottom of the container so that it solidifies.
The machine comprises also a modelling plate suited to support the solidified layers of the three-dimensional object, associated with actuator means suited to provide for moving the plate in a direction perpendicular to the bottom of the container.
According to a stereolithography method of the known type using the above mentioned machine, first of all the modelling plate is arranged at a distance from the bottom of the container that is equal to the thickness of the layer to be solidified.
Successively, the liquid resin layer adjacent to the bottom of the container is selectively irradiated by means of the light source in order to solidify it.
The modelling plate is configured so that the solidified layer adheres to it while, in the contrary, the bottom of the container is provided with a covering that reduces such adhesion.
The modelling plate is successively moved away from the bottom of the container, so as to make the solidified layer emerge from the liquid resin and thus make it possible to restore the thickness of the liquid resin that is necessary for processing a successive layer of the object.
In fact, lifting the modelling plate and the solidified layer means leaving a depression in the liquid resin, which is filled by the spontaneous flow of the resin itself.
Said levelling action restores the thickness of the liquid resin that is necessary to solidify a new layer of the object and furthermore prevents air bubbles from remaining trapped into the liquid resin during the successive lowering of the modelling plate, which may affect the integrity of the successive layer of the three-dimensional object.
Once said spontaneous levelling is completed, the modelling plate is immersed to again in the liquid resin and a further layer of the object is solidified.
The method described above poses the drawback that the overall processing time of the three-dimensional object is considerably prolonged due to the waiting time needed after the solidification of each layer of the object for the liquid resin to level out.
Since the number of layers that form an object obtained through stereolithography can reach several hundreds, it can be understood that the waiting times mentioned above result in a considerable increase in the processing time.
Obviously, the waiting times are proportional to the viscosity of the liquid resin. Therefore, the drawback mentioned above is particularly important when resins of the so-called “hybrid” type are used, which comprise particles in ceramic or other materials mixed with the polymeric component.
Said hybrid resins are suitable for producing objects with high mechanical resistance but on the other hand their viscosity is much higher compared to the other resins commonly used in stereolithography.
A known embodiment, described in the Patent application WO 2010/045950, includes a movable tank and a fixed levelling paddle arranged in contact with the resin.
Before the solidification of each layer, the tank is moved so that the levelling paddle fills the depression left by the modelling plate when it is lifted.
This embodiment poses the drawback that it requires a certain amount of space for the movement of the tank that, consequently, involves an increase in the overall dimensions of the machine.